This invention relates generally to the fabrication and composition of superconducting material and is particularly directed to the production of a new class of superconductors and superconductor precursors. The precursors of this invention relate to glassy materials which are 100% dense and can be converted to superconductors having a vastly superior density, not heretofore available. These superconductors have large current densities and also permit fabrication of superconductors in a variety of physical shapes.
This invention relates generally to the fabrication and composition of superconducting materials and is particularly directed to the production of very dense superconducting material and of glass precursors. New shapes are also possible with the invention.
Certain metals, alloys, and chemical compounds known as superconductors exhibit zero electrical resistivity and complete diamagnetism at very low temperatures and magnetic fields. The transition of a metal from normal electrical conducting properties to superconducting properties depends primarily on (1) the temperature and (2) the magnetic field at the surfaced of the metal. The superconductive state of the metal exists for temperatures less than its characteristic critical temperature, T.sub.c. The most practical superconducting materials exhibit very low critical temperatures, i.e., on the order of 4.degree.-23.degree. K. However, recent developments have produced new superconducting materials, based on oxides, having critical temperatures on the order of 100.degree. K.
Superconductors also exhibit a characteristic critical electric current density, J.sub.c, measured in amps/cm.sup.2. By increasing the current density in a superconducting material to its J.sub.c characteristic value, it can be driven into a normal conducting state. Thus, the current density at which this transition occurs is termed the conductive material s critical current density. It is of course desirable for a superconductor to have a high critical current density to allow it to conduct large currents while remaining superconductive.
Since the report of superconductivity above 30.degree. K in a multiphase La-Ba-Cu-O system, a great deal of activity has centered on the isolation of superconducting phases, the determination of their structures, and the search for other high temperature superconductors. Until recently, there were only two oxide structure-types which demonstrated this high-T.sub.c behavior, the tetragonal form of La.sub.2-x M.sub.x CuO.sub.4 (M=Ca,Sr,Ba) and the YBa.sub.2 Cu.sub.3 O.sub.x distorted perovsite. Superconductivity in the Bi-Sr-Cu-O system was first reported for Sr.sub.2 Bi.sub.2 Cu.sub.2 O.sub.7+.delta., where T's in the range 7K to 22K were observed, but the structure of the single phase material remains unknown. The production of Bi, Sr, Ca, Cu.sub.2, O.sub.x superconductors with onset of 80.degree. K or 105.degree. K has proven difficult. The phase responsible for the lower T.sub.c (about 80.degree. K) can be synthesized over wide cation starting compositions, although obtaining the 110K transition has been a problem. Essentially identical x-ray powder patterns are produced from different metal-ion starting ratios. There are now several reports claiming different single phase materials as responsible for superconductivity in the Ca-Sr, Bi-Cu-O system.
The standard method of sample preparation is to form intimate mixtures of the constituent binary oxides and carbonates. Historically, this route is chosen because of previous difficulties in synthesizing YBa.sub.2 Cu.sub.3 O.sub.x ("123") using alternate techniques. For example, heating the "123" samples above their melting points (approx. 1030.degree. C.) results in multiphase samples which are not superconducting. Samples in the Bi-Sr-Ca-Cu-O system, when produced by powder processing, are low density and suffer from lack of sintering. SEM photographs of crystallites of the superconducting phase clearly demonstrate their plate-like morphology. This feature, combined with the very low packing density of these plates (about 40%) suggests that powder methods may be very inefficient as a synthetic route.
We have synthesized these new phases from amorphous glasses which are produced by quenching melts of the constituent oxides, followed by firing to produce the superconducting phase. This method can produce 100% dense starting materials which eliminates the problem of long range diffusion during synthesis. This technique may permit the synthesis of pure materials with enhanced bulk properties, such as optimal critical currents.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.